text:
What are the effects of crosswind on glide angle?
When gliding what are the effects of a crosswind on my glide path angle?
If I have head-wind, the minimum flight path angle (usually called gamma) will be lower than the one without crosswind and with an attitude with a larger lift coefficient  Cl than lift coefficient of max efficiency with no wind?
On the other hand, if I have tailwind gamma will be greater and the Cl will be lower than Cl of max efficiency. Briefly if I have tailwind without engine I can glide much longer, is it right?

You seem to have two questions unrelated questions - your first sentence and your last. They are both valid, but would probably be better being separated into two posts so they can be addressed individually.

your use of 'crosswind' and 'headwind' and 'tailwind' makes it hard to understand what you're asking

I changed horizontal wind to crosswind, so I may have misunderstood the original intent and messed things up. Feel free to undo that if necessary.

@FreeMan horizontal means that it simply has no vertical component. crosswind is more restrictive: the main wind direction must not be parallel to the direction of travel. Given the original intentions (the example is for headwind) I would have left the original formulation.

That makes sense, @Federico - my changes were based on the fact that OP specified head wind, tail wind, and horizontal wind, so I assumed he meant cross wind. Again, anyone is free to revert/reedit without hurting my feelings.

Federico is right, i did mean horizontal wind with no vertical component and parallel to the direction of travel.

@Davide For purposes of this question (and my answer below) the vertical component of wind can probably be ignored: it will have an affect, but that effect will simply be to increase or decrease the descent rate by whatever that vertical component is. With the exception of convective activity in/around thunderstorms and the kinds of lift gliders rely on it's probably less interesting than the horizontal (head/tail/cross) wind components for our purposes.

Wind affects your flight path angle by affecting how far you travel while you're descending.
With no thrust from the engine an airplane is falling out of the sky at a roughly constant rate of descent. The wind direction affects the groundspeed, and thus the distance covered between when the engine failed & when the aircraft hits the ground.
This is more directly observable from outside the airplane.
Consider these 4 scenarios on a post-it note: The airplane has lost its engine, and the pilot is flying at best-glide airspeed while holding a constant magnetic heading (making no wind corrections):

No Wind
The aircraft descends along the no-wind glide path (about 45 degrees in my drawing), and travels in a straight line over the ground (the "ground track") while doing so.
Headwind
The aircraft descends along a steeper glide path (about 60 degrees in my drawing), because while it's still descending at the same rate the headwind is slowing its progress over the ground. Since it's a direct headwind (right on the nose) the ground track is still straight.
Tailwind
The aircraft descends along a shallower glide path (about 30 degrees in my drawing), because while it's still descending at the same rate it has a tailwind, which is making it move faster over the ground. Since it's a direct tailwind the ground track is still straight here too.
Crosswind
The aircraft descends along the no-wind glide path (because it has neither a headwind nor a tailwind), but the crosswind pushes the ground track (in this case into the inconveniently-placed tree).

Your endurance (time aloft) will be similarly unaffected: If you've set the aircraft up at its optimum glide angle-of-attack (best glide airspeed) the descent rate is pretty much the same regardless of whether you have a headwind, tailwind, or crosswind. You will fall the same distance in the same amount of time, it's just a matter of how far you travel while you're falling (which determines where you land).
You can test this for yourself in an airplane by doing engine-out drills: Line up with a headwind, pull the throttle out, and pitch for the aircraft's best glide speed. Note your vertical speed (rate of descent), and then repeat the test in the opposite direction (tailwind) or at 90 degrees to your original course (direct crosswind): The vertical speed will be effectively the same.

In general, wind does not affect the aerodynamics, it mostly affects where you land. So, for example, if you have a tail wind your glide time will be the same, but you will travel farther than if you had a head wind.
As far as the "minimum flight path angle", the angle of maximum drag, that is largely irrelevant to actual flight because you never (intentionally) fly at that angle. When gliding, you usually aim at the best glide speed which will take you the farthest, or occasionally at minimum sink rate rate if you want to maximize your time of flight and distance is not a factor.

Why is the minimum flight path angle the angle of maximum drag ? So you're saying that that maximum range is obtained with max velocity ? And forgive me, what is the skin rate ?

@Davide I assumed you were aeronautical engineering student since minimum flight path angle is a technical term that I have never seen used except in highly technical aeronautics. In any case if you refer to an aeronautics textbook it will explain the term, which is complicated. The maximum range is attained with the best lift-to-drag ratio. The sink rate is the distance of vertical descent per unit time.


text:
Why are some windows in cockpits made openable?
Why do aircraft allow sliding / opening windows in cockpits? What purpose does it serve, and why the same can't be done for passenger windows as well?

I always assumed some airports must have drivethrough mcdonalds for pilots on tight schedules.

Now for the non-serious answer/comment: https://www.youtube.com/watch?v=-5viPnzK-S4   "I gave (your keys) to the pilot and he threw them out the window"   <-- Ansett Airlines advertisement in New Zealand in the 90s.

@Criggie : Thanks for the link..

Could you please edit question and source the image ?

How can you believe that letting passengers open their windows is remotely feasible?

Pilots know better than to open a window when the cabin is pressurized.  They can be trusted to be with responsible use of the ability to open a window, while passengers cannot.

@kebs : You want the source link for the image? Just right click and find out

@Firee The link on imgur is not a relevant information. What I meant is that as soon as you post an image on a SE site, there "might" be some copyright issue (I'm not saying its the case here). So to be able to clarify this point, a good practice when posting an image is to source it, example here. See here for more.

Of course, if your are the photograph, then this doesn't apply...

@kebs: The question on image copyright was vague, and also that is not the official SE guidelines. Nonetheless, I have not re-hosted or uploaded the image anywhere else, I have just linked the original url from Instagram. However, now that I hover over the image here on SE, I see an imgur link. How did this happen?

that is not the official SE guidelines: true, you don't need to do anything. How did this happen? Unclear, you didn't actually edit the question, so nothing could have changed...

My guess is that Captain Bob Hayes is British, and that the Queen has bestowed upon him the title Order of Nuttiness?

@Firee - That's why I asked Must we cite pictures from the internet? I'm pretty sure imgur cashes uploaded and linked pictures.

The opening cockpit windows do serve some purpose, like providing an additional means of evacuation during an emergency.* There have been instances where the crew had used the cockpit windows to escape during hijackings.
There are other purposes like assisting in crew inspection, signaling ground crew, additional ventilation, smoke egress etc (It can also be used for throwing things out, though it is not generally recommended). One main reason (atleast in the past) appears to be to improve the pilot visibility in case the windshield becomes opaque, for example.
The windows are not found in all aircraft (for eg. 787 doesn't have it) and are not very easy to open (note that it opens inwards). From USAToday Ask the Captain:

First there is a locking mechanism, then either a crank which moves the entire window assembly including the frame, or a lever with which to move the window assembly. When the window is closed and locked, it is sealed shut. When open, it does not separate from the frame as the window in your car does.

Actually, FAR 25 Section 773- Pilot Compartment View requires cockpit windows to be openable (or some other means to have clear windshield during precipitation) :

(b) Precipitation conditions. For precipitation conditions, the following apply:
(1) The airplane must have a means to maintain a clear portion of the windshield, during precipitation conditions, sufficient for both pilots to have a sufficiently extensive view along the flight path in normal flight attitudes of the airplane.
(3) The first pilot must have a window that—
(i) Is openable under the conditions prescribed in paragraph (b)(1) of this section when the cabin is not pressurized;
(4) The openable window specified in paragraph (b)(3) of this section need not be provided if it is shown that an area of the transparent surface will remain clear sufficient for at least one pilot to land the airplane safely in the event of—
(i) Any system failure or combination of failures which is not extremely improbable, in accordance with § 25.1309, under the precipitation conditions specified in paragraph (b)(1) of this section.
(ii) An encounter with severe hail, birds, or insects.

In a pressurized cabin, providing windows is not a great idea. (In addition to the increased weight), if anyone manages to open the window, it would be a recipe for disaster (depressurization -> oxygen masks -> land).

*Many aircraft have escape ropes mounted in a compartment above the window on each side of the cockpit as in this A320:

Photo source
And a video of it being used.

Added a photo and a video link. Hope you don't mind.

@TomMcW Good one. Thanks..

I was once riding in the jumpseat of a commercial airliner and the captain seemed to think that these windows were so he could smoke!  As soon as the engines were running, so there was pressurization, he opened the window and lit a cigarette.  He did the same thing as soon as we landed.  With the window opened, I could not smell his cigarette in the cockpit, because the pressurization system did such a good job at removing the smoke.

Well now it looks that the openable window does come in handy, wonder why Boeing left it out in the 787.

In the Pilatus PC-12, the pilot's side has a "DV window", a "direct view windows," that is hinged at the back and opens so the pilot has an unobstructed view out forward of the aircraft.
This window is specifically designed to allow the pilot to land an aircraft when the windshield is iced over, and the de-icing equipment is INOP.

Other answers provide good broad picture information. Let me add a specific example.
The windows in my work aircraft open, and we have specifically modified them to open fully out of view and remain open. Windows that open allow me to better perform my job. Note that my aircraft is non-pressurized.
One of the things we often do is drop messages to people on the ground. The open window provides a means of jettisoning that message package.
Aerial photography and observation of the ground is in integral part of what we do. Being able to open the window to provide an unrestricted view can give better photo or viewing environment. Conditions such as sun glare render this an important feature.
Finally, being able to open the window on a hot day can be a welcome addition to the ventilation system.

Very cool.  I'd wager your work airplane does not have two engines on each wing like the photo in the question :-)

@Dan You would win that wager :)

and why the same can't be done for passenger windows as well?

Anybody who opens even a bit a potential airliner sliding window near his seat creates in the entire plane the conditions existent on the top of Mount Everest, even worse. Nobody wants to stay in such an environment. In conclusion passenger sliding windows serve no purpose.

That should be a comment, as it doesn't answer the main question which is about cockpit windows and the difference with other windows. That said, good point!

For any reasonable design, opening the window would be impossible while the cabin is pressurized anyway. The bigger problem is making sure that all of the passengers actually close and securely lock them before the cabin is pressurized and leave them that way. As good as passengers are at following instructions, I'm sure you can see how this would be a problem. Furthermore, it creates ~100 additional potential points of failure, since any window failing to secure would ground the aircraft. And, of course, it also adds a non-trivial amount of weight.

The difference between the inside and the outside pressure is max 0.65 atm which means 67 $kgf/dm^2$. With a mechanism like the one used by car windows it is easy to slide a 10 x 10 $cm^2$ window pressed by 67 kgf. Anyway, the biggest issue is that sliding passenger windows serve no useful purpose for the reason I have already explained.

@reirab So many doors have to be secured before takeoff, so I am sure securing the windows before takeoff should not be an issue. Plus think of the advantages of openable windows in case of a crash where people die of asphyxia.

@Firee The cabin doors are secured by trained cabin crew who cross-check each other. The cargo doors are secured by trained ground crew (and accidents still happen.) Passengers are not trained crew and are notoriously bad at following instructions.

@RobertWerner That's why I said "for any reasonable design." They'd need to be designed to be pulled inwards to open, like the flight deck windows. Also, cabin windows are much larger than 10 cm x 10 cm. They range from about 31 cm x 23 cm to 47 cm x 27 cm (the latter is the 787.) That would be a total force of 1,085 lb (488 kg) to 1,930 lb (876 kg.) Good luck pulling that in. Also, cracking a window would not produce Everest-like conditions in the entire cabin. Air would be rushing out, not in. The outflow value would be able to regulate the pressure for a small crack.

If you open the window just a bit then why you have to slide an entire 47 cm x 27 cm window? A smaller movable window is enough. 2) Also the plane might be able to regulate the pressure if a single window is opened a bit it will not be able to do the same thing if many windows are opened and the pressure will drop at levels where people can not survive. Also the first part of the question "Why are some windows in cockpits made openable?" was justified. The second about passenger windows that could be opened was really dumb. I can not explain the large no of pluses. It is a mystery.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
Can air travel cause packaging air pads to burst or deflate?
This question is a new cross-post from the physics stack exchange: https://physics.stackexchange.com/questions/265884/can-a-decompressed-airplane-cargo-hold-cause-packaging-air-pads-to-burst-defla
I received a parcel from Germany to Chile. The contents were severely bruised due to a lack of packaging material. On second inspection I found a bunch of deflated air pads.
It was a personal shipment, the air pads were used and potentially not in pristine condition, but there were several brands of them, a manufacturing defect can be ruled out. It seems most of them burst, while at least one still had some air remaining and just had a leak that required a bit of pressure to release more air. Surprisingly there was very little air remaining, maybe 20% of the maximum volume.
The contents were light, I doubt they could have damaged a pad at all, let alone destroy them all. Could this phenomena be caused by a pressure difference between the air inside the packs and the cargo plane's hold? It seems a bad explanation considering the popularity of the packaging material.
I did check for needles and dead chicken. ;-)

Possible duplicate of Are cargo holds pressurised these days?

Re duplicate: This is a more general question along the lines of: How can this happen? The answer may very well be: Probably not in a plane. The suggested question can be a great follow up, if air pressure can be the answer.

It's not bubble wrap: http://www.storopack.us/en/products-solutions/flexible-protective-packaging/airplusr-inflatable-void-fill-and-cushioning.html

It's basically the same stuff as bubble wrap, just different size bubbles. Same physics.

True, though I think bubble wrap is stronger structurally with smaller air pockets and softer plastic.

It might be better to ask this of Physics.SE

This also got closed as off-topic in physics. Bottom line would be, the question has no place on stackexchange.

Simple answer: yes, air travel can deflate or pop air pillows.
Longer answer: Cargo holds are at the same pressure as the rest of the airplane, it's part of the pressure vessel. Generally airplanes are pressured to about 8000ft, so the air is a lot thinner and therefore there will be a higher pressure differential between the air in the pads and the air in the hold. Thin plastic is somewhat porous and will leak some air, the rate this happens will be faster when there's a higher pressure differential. This differential is known to pop these air pillows to the point many will not use them when shipping by air. Styrofoam, cellulose pops, or crumpled paper are the preferred method.
You can see these forces at work next time you fly yourself by taking an empty disposable plastic water bottle on board. Seal it with the cap at airport altitude before departure. After takeoff as the airplane climbs the plastic will bulge as the cabin pressure drops, and when you uncap it you will probably hear a hiss. Seal it back up and then during descent the bottle will crumple as the air pressure increases.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How to find past flight routes between 1977/82 Latin America and the former Soviet Union?
I hope now that I am in the correct place to post my question, as I have been kindly accompanied to the exit on travel part of stack overflow. ;)
Is there a way to find past flight routes between Panama City and St. Petersburg (former Leningrad) in the years 1977 to 1982?
To be precise: over which flight route would someone reach to either from Panama City to St. Petersburg and vice versa in the late 1970s?
I would need this information for an historic background check up in my genealogical (family tree) research.
Bonus point: I have heard that especially in Latin America, flights back then would "collect" passengers (part of student exchange programs between Latin America and the former Soviet Union) from capital to capital before the plane would go into the intercontinental flight, e. g. start in Panama City, going to San José, Costa Rica, to Managua, Nicaragua, to Belize City, Belize to ... to Havanna, Cuba. However, I am not sure about this claim.

While someone here may answer, you may also try on PPRuNe, this is a site of pilots and former pilots that may have flown such routes.

Here are a series of old Aeroflot schedules. In 1977, from Latin America, they flew two scheduled routes

Lima > Havana > Lisbon > Frankfurt > Moscow
Mexico City > Havana > Lisbon > Frankfurt > Moscow

It seems unlikely there were scheduled stops not listed, as if you're including a 1h10m refuelling stop at Lisbon you may as well list them all :-). Note that most of the international flights (perhaps all the long-distance ones?) went into Moscow; there were a few into Leningrad but all European routes.
However, it is possible that Aeroflot provided a charter-type service to collect foreign students (especially if it was some kind of large-scale state-backed scholarship scheme). In which case... the "hopping" route and then to Leningrad (via a fuelling stop somewhere) seems quite plausible, but finding documentation on it forty years later would be incredibly challenging.


text:
Why does the Canadair Regional Jet still use CRT screens?
All the CRJ aircraft from the 100 up through the new 1000 use the antiquated CRT technology for the main primary and secondary displays as well as the ED1 and ED2 displays.
Aren't flat screens a more modern, cheaper and reliable technology? Why aren't there flat screen retrofit units for the older aircraft or flat screens in the newer aircraft?

If they work, why replace them? Its an added expense with not much benefit. Are you sure that the newer ones don't use LCD technology? It could also be that they have an approved avionics platform and don't want the expense of going through the FAA/CAA approvals again.

The pictures and videos of the CRJ-1000 I've seen feature LCD screens on the RTUs and FMCDU, but the 6 main screens are still CRT. The old CRT screens are super heavy and require significant cooling. Those familiar with CRJ procedures know about the cooling requirements and associated limitations. I suspect that building these CRTs is more expensive than building LCD screens to replace in terms of the unit. Perhaps certification changes that equation.

@RonBeyer: You replace them now so you don't have to worry about suppliers in 2 years time. The problem is not engineering but logistics. AFAICT, FAA certification is even stricter than medical certification, when I look at the speed of introduction of new technology.

@MSalters you'd think. In my experience quite often you just buy up all the stock you can get if it looks like a manufacturer will stop production and hope it lasts until you can get a replacement for the entire platform.

I know the exact answer, even if 2 years late.
Firstly, Collins makes their own displays.
Secondly, it was a conscious decision during CRJ700 development to remain CRT, solely for the reason that airlines already had spares supplies of the CRT displays for the CRJ200 fleet. Changing to LCD in the 700, 900, and 1000 would require two different spares supplies for operators of the entire range.
Additionally, it was a cost matter... the PL4 was already certified, and the airlines :

want the new plane yesterday, and
don't want LCD certification costs passed along to the final price.

Welcome to Av.SE!

Welcome! Your answer sounds very reasonable - do you have any sources to back it up? That would add tremendous credibility.

Hey Freeman, sorry for the delay... My source is a) I was a captain on a crj900 for 7 years, and B) was doing a flight with the Fleet Manager and this is what he told me.

This is just an educated guess, but it was too much info for comments. Maybe someone can come up with a specific answer, but it seems it's probably a certification delay. The CRJ's all use the Rockwell Collins Pro Line 4 avionics suite, which uses CRT's. In 2000 Rockwell Collins introduced the Pro Line 21 suite which uses LCD screens. They have been slowly introducing PL21 retrofits for aircraft with PL4 as they receive STC's. The Falcon 50 and 2000 began getting retrofits in 2008, followed by Beechjet 400A in 2009, King Airs in 2010, Learjet 60 and Gulfstream G100 in 2012, Piaggio Avanti in 2013, then Hawker 800 and Challenger 300 & 605 in 2014.1
My presumption is that the CRJ's have a type cert with the PL4 and they very well may be working on getting a cert with the PL21 or the newer Pro Line Fusion suite. It will certainly become increasingly difficult to find replacement CRT's as there a only a couple of companies still manufacturing them. It seems just swapping display types is not feasible with the PL4 system. The PL21 system was designed to be more flexible so it's easier to integrate improvements in technology.2

1 These dates might be wrong. I'm finding different dates from different sources. For example, Falcons began getting them in the US in 2006 but it didn't receive EASA certification until 2014.
2 http://jetadvisors.com/rockwell-collins-proline-21-avionics/

True. Certification costs are enormous, and unless an airline is willing to pay for it, the avionics supplier won't do it. If things are working just fine, airlines would rather spend their money on other priorities. People assume because the name sounds cool that it's cutting edge, but most of Avionics is not cutting edge. There are some cool things being developed, but they take time to find their way into flyable solutions, and there is a balance in the retrofit market between a functional status quo and the new features an upgrade could bring. It's really up to the airlines to decide.

@KentA. You may be right, since all of the aircraft i mentioned are popular with private owners, whereas there probably aren't too many CRJ private jets. I doubt too many of the regionals will go back and retrofit their fleet. Plus they usually want all their aircraft to be the same. So that part is really pure speculation. It boils down to the older system being designed and certified with CRT's and they prob can't just swap the displays. They can't continue indefinitely, though, because CRT's are disappearing fast. Maybe Rockwell Collins was proactive and bought 40 years with of them :)

The big avionics producers do in fact have stockpiles of hardware. They have modern systems running on modern hardware, but their customers are the ones who decide whether they are willing to pay for a retrofit (and the associated certification activities). So the producers buy up and store large quantities of components in order to provide maintenance for as long a time as possible. But you are correct, at some point, replacement parts will be exhausted, and the airline will have to buy a more modern system - and their supplier will be happy to sell their newer stuff to them.

I have read somewhere that there is only one company that manufactures CRT screens now and CRT screens will have to be replaces at some time in the future because there will be no spares. I think work is in progress to certify them.

Yes, the company is Toshiba and production has ended.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What alternative methods are there for controlling a plane when flight control surfaces have failed?
I saw a TV show once about Japan Airlines flight 123, in which the aft pressure bulkhead ruptured, disabling hydraulics and ripping off the vertical stabiliser. So the flight crashed after some time of flying with up-and-down oscillation.
Since then, I've also heard of two flights in which the aircraft lost control of flight control surfaces, but pilots guided their crippled plane into a landing using engine throttles only:

United Airlines flight 232
2003 Baghdad DHL attempted shootdown incident, in which a cargo plane in Iraq was hit on a wing by a missile

In principle, could such a method have been used in Japan Airlines flight 123?
In principle, would it be possible to control such a plane by use of cooperating passengers moving around in the cabin, e.g. by instruction of a pilot, to shift the weight balance of the aircraft?

Simon says, "Everybody run to the front of the plane!"  Okay, now, "Everybody lean to the RIGHT !!!"

I'm sure one of the engine throttle only landings was United 232.  What was the other one?  I can't recall it.

@SteveV.: 2003 Baghdad DHL attempted shootdown incident, in which a cargo plane in Iraq was hit on a wing by a missile. I've edited the question

I would say that without a vertical stabilizer, their chances are close to zero due to to loss of lateral stability. I strongly suspect that the loss of the tail makes the plane's adverse yaw much worse thus making control extremely unintuitive (not sure if it's even possible to turn since any attempt to roll into a turn would make the nose point in the opposite direction)

Magic.  Most definitely magic.

Control means to be able to vary the forces which act on an airplane, both longitudinally and vertically. Ideally, any variation will create an immediate feedback, so the pilot can "feel" how much more action is needed for the desired change. If, however, the variation of forces builds up slowly, the dynamic answer of the airplane will make the closing of the feedback loop harder to achieve. The pilot might either overcontrol and get into an oscillation, or he/she will be too careful, and the desired change never happens or happens too slowly.
Regular control surfaces are small, so they can move quickly, and their forces act on a long lever arm, so the control effect is large. Also, control surfaces change lift proportional to their deflection, so predicting the consequence of an input is easy and the feedback can be felt immediately.
Even if the primary control surfaces are ineffective, several ways of control still exist. I will list them in order of reaction speed:
The elevator can be replaced by:

Pitch trim,
Wing flaps,
Thrust variation, if the engines sit above or below the longitudinal axis of inertia,
Weight shifting, if fuel can be moved between tanks or payload can be shifted longitudinally.

The last two options work very slowly, and while they can be used to limit sink speed, it will be extremely hard to avoid overcontrol and oscillations. Situations which need precise pitch control like landings are impossible to master for an unprepared human pilot when only such crude means of control remain.
The rudder can be replaced by:

Rudder trim,
Ailerons plus pitch control in case of agile aircraft with low aspect ratio,
Asymmetric spoiler (if available),
Asymmentric thrust.

Again, the last option is rather slow and cannot be used for precise directional control.
The ailerons can be replaced by:

Rudder plus dihedral effect,
Asymmetric spoiler (if available),
Sideways fuel pumping.

Here again the last option is too crude and too slow for maneuvering.
If the aircraft is otherwise undamaged, and the flight dynamics are well known, a specially programmed autopilot could fly an aircraft even when using the last listed options, because it can much better predict than a human pilot how much effect its actions will have. Humans rely on a feedback loop for control, and this is not working anymore if the reaction time of the system is far above its eigenfrequencies of motion.
Note the "if": How likely will it be that only all control surface actuators have failed, but the aircraft is intact otherwise? In most cases, the control failure is a consequence of another failure which will alter the dynamic response such that an autopilot would be unprepared. There are experimental autopilots which can adapt, but they are only used by the military so far.
With the loss of most of its tail section, JAL123 would have been hard to control even if the equivalent of full rudder authority would have been given to the pilots. With thrust only, fighting the inevitable dutch roll motion by hand is impossible.

Note that if you have a total failure of the primary flight controls, this likely indicates a total failure of the plane's hydraulics, in which case you won't have any trim, flaps, or spoilers to work with, either - you'd be limited to fuel transfer and throttle steering.

@Sean Except for cases where trim is electric. That is not so uncommon.

As far as I understand, the JAL123 pilots did use thrust manipulation in an attempt to control their crippled aircraft -- however, Japan's largely mountainous terrain conspired against them, as trying to figure out fly-by-thrust-lever requires a massive quantity of airspace, and unfortunately for JAL123, there was a mountain in the way of that effort.  UAL232 and OO-DLL, on the other hand, both happened over relatively flat terrain, which meant that the aircrews could concentrate on working with their crippled airplanes to get them back on the ground as safely as possible, without having to worry about terrain clearance.
Of course, the missing vertical stab didn't help them either, as it meant they had to do much more work to control their airplane.

The missing tail on JAL123 would have severely affected the aircraft's stability. I doubt if anyone can land (or crash land) on a runway using throttles only in that airplane, even if there were no mountains in the vicinity.

It should be mentioned that as a part of the investigation, pilots tried simulating JAL123's last 32 minutes: No one could keep the plane in the air for the full 32 minutes.

Actually, the timing is also really important: Pilots became aware that total loss of controls could happen, so some started training for it after the incident, thus giving better chance of success after JAL123

I don't know the precise directional-stability characteristics of the 747, but keep in mind that, for some aircraft at least, losing the vertical stabiliser does not always necessarily preclude a safe landing (or "landing").

As others have stated, it is possible to control a plane using throttles alone, assuming the control surfaces are largely neutral and the plane is at an stable attitude (e.g. not spinning or stalling):

To climb, increase throttle of all engines
To descent, decrease throttle of all engines
To turn, decrease throttle of engines on the inner side of the turn

Such feat would be very difficult as flight crews receive no training of such control methods.

On a large airliner, shifting weights would produce little effect, since the weight of passengers contribute only a very small portion of the entire weight.

NASA has already shown that it is technically possible to develop a system to automatically control a plane using throttles. Not only does the system provides stable and acceptable approaches and landings, it was also tested in unusual attitudes and at speeds 100 knots above approach speeds.
As to why this kind of system is not implemented across commercial airliners, the usual answers apply...

It's also debatable whether such a system could be implemented a typical airline transport jet.  Military jets practically fly on thrust alone (http://theaviationist.com/2014/09/15/f-15-lands-with-one-wing/).  Airliners still depend very much on their wings, ailerons, elevators, and rudder for stability.  Although UAL232 provides some anecdotal evidence that it might be possible, a lot would depend on the nature of the situation causing the emergency.

Don't put down the NASA report that easily. The F15 considered has its engines close together. That means it's harder to turn as the torque arm is a lot shorter. The F15 also predates thrust vectoring, so it's similar to commercial planes in that respect as well. But yes, you do need two wings - that's not waht this question was about. A wing usually isn't considered as a flight control surface.

Airliners have got into trouble from too many passengers rushing to one side or the other to watch something happening there. Things like a volcano erupting, a space shuttle launch. Rarely something that can't be corrected for, but if the normal controls are dead... I'd guess if done deliberately it might be used as a control system, but it'd be hard to control.

@MSalters  I wasn't really "putting down the NASA report", just saying that the nature of the emergency would govern the effectiveness of trying to control an airliner with thrust alone.  It might be one of those situations where while technically possible, the benefit may not be there given the fact that the thrust ratios are way different between the two types of plane, and the importance of the control surfaces differs significantly as a result.

The question here is very much related to the way in which the controls were to stop working. If the surfaces are in a more or less neutral position it would be possible to control the airplane (to an extent) using the engines based on the principal that more thrust would increase speed and create more lift as well as less thrust creating a decent. You could to some extent use differential engine power to turn as well.

In principle, would it be possible to control such a plane by use of cooperating passengers moving around in the cabin, e.g. by instruction of a pilot, to shift the weight balance of the aircraft?

Maybe, this depends largely on the plane but basic aircraft training would say yes. This is largely why you have to do weight and balance when you learn to fly. The issue here would gingerly moving everyone around so as not to put the plane in a stall.
Keep in mind that with Japan Air 123 the vertical stabilizer being blown off was a big issue...

On a passenger airliner, though, passengers are a rather small percentage of the overall weight. I would think they could probably make some difference in pitch, but I would think any difference in roll would be negligible, since the arm length would be rather short, especially when compared to the arm length associated with the weight of the fuel in the wings (and the engines, if wing-mounted.)

yes this is true and I would agree. But it has been noted here that passengers are negligible when it comes to weight. So it might not have an effect anywhere.

If the control surfaces fail due to the hydraulic lines/control cables being severed (rather than, say, jammed actuators), as with JAL123 (well, most of the control surfaces on JAL123; the rudder stopped working by dint of no longer being attached to the aircraft), the aerodynamic loads on the surfaces should force them back to "a more or less neutral position".  (1\2)

With one exception: on some aircraft, under at least some flight conditions, the aerodynamic loads on the elevator might not be enough to keep it from dropping down under its own weight, in which case you're basically screwed.

Fly-by-wire control systems may be quadruplexed such that there are 4 channels. This is all a part of the redundancy that is built into aircraft.
Also, take a look at this article about the A380 and redundancy.
http://www.roger-wilco.net/would-i-fly-on-an-airbus-a380/
A380 redundancy

What are those boxes on the right side of the fuselage powered by the green system?


text:
What does M mean in A400M Atlas?
Atlas was the guy in Greek mythology that was literally carrying the world/planet in his shoulders. But was does "M" mean? Military? That's a guess, though...

That’s a good guess

I've never seen any mention of a specific meaning in EADS documentation, but as it's marketed exclusively (afaik) to military operators that's likely a correct assumption.

@jwenting indeed, only military operators to my knowledge too!

Without the M, people would simply confuse the military cargo plane with the civil airliners that Airbus builds (e.g. A320, A380, etc).

I thought of that as well. But I was hoping to find a reference.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Is there a maximum airspeed to deploy a ram air turbine?
Is there any airspeed limit for deploying a ram air turbine? Can airspeed be determined from the rate of its propeller rotation? Could that data be used to determine altitude?

Why should there be vertical speed limit? The turbine only cares whether it is facing the relative wind, not how it is oriented.

I refuse to vote for close as I am hoping for it to be a typo as that question would be interesting if the horizontal speed is considered...

Does he mean the resistance of high wind to avoid the mechanics to drop it out?

Yes, this could be an interesting question if the OP clarifies a little about what he's looking for. I would assume that any answer (to the first part, at least) will be type-specific.

Voting to close - as Reirab and Maverick mention, there is certainly a potential good question hidden away here somewhere, but right now I don't see it. Please feel free to edit this question and then request a re-open.

Looks like a good question now. Hope someone comes by with a response or two.

Is there a maximum airspeed to deploy a ram air turbine?

The ram air turbine (RAT) can be extended and used at any speed above a minimum speed, because under this minimum speed, power is difficult to generate.
Generated power may be decreased below 130 knots, and extension is prevented below 100 knots because of additional drag produced for nearly no power obtained.
The RAT can be extended automatically (loss of engines or pressure) and manually, e.g. logic for RAT extension on A330 (note the inhibition by airspeed below 100 kt and by on-the-ground sensor:

Source: Airbus A330 FCOM - Hydraulics

Can airspeed be determined from the rate of its propeller rotation?

No, the RAT turns at constant speed to produce a constant AC frequency or hydraulic pressure or both. The rotational speed is stabilized by a governor which adjusts the angle of attack of the blades according to airspeed.
If the governor fails to maintain the speed, there is a security to prevent overspeed and blade damage/release.

Emergency Airplane RATs, Michael J. Zolidis, Hamilton Sundstrand, 2006.

A ram air turbine from Hamilton Sundstrand has a mechanical speed governor to vary the RAT blades angle according to the airspeed, and maintain a constant rotational speed.
At low speed, the blade angle is maintained near-perpendicular to the airflow to extract the most energy possible. When the ram speed increases, the angle is progressively reduced.
HS seems to measure the RAT power at 130 KEAS (Knots Equivalent Air Speed), e.g. the A380 RAT is 62” (1.57 m) diameter and delivers 85 HP (63 kW) at 130 KEAS. This mean at 130 KEAS or above, the full power is delivered.

Uncommanded Ram Air Turbine (RAT) extensions - Airbus

The RAT auto-deploys when both engines fail, or manually from a panel overhead. It's prevented to deploy on the ground, and below 100 KCAS
The speed value is provided by the ADIRU.
The RAT is able to be extended in all flight envelope conditions.

Embraer 190 - Airplane Operations Manual

There is no maximum altitude or speed for RAT deployment
Deployment occurs whenever AC buses are not powered or manually.
RAT deployment takes 8 seconds.
RAT has its full capacity at 130 KEAS or above.
Below 130 KEAS, only the AC bus is powered. DC buses are powered by the batteries, as well as the standby AC bus through a DC/AC converter.

ETOPS CAA

For ETOPS, three electrical power sources are required. If the RAT is one of them, it must be demonstrated as reliable (extension and generation).
The extension mustn't rely on engine power :-)

The most popularized RAT use may be for AC143 Gimli Glider, with a dual engine loss.
More:

RAT article on Wikipedia.
A320 RAT extension and retraction on Youtube.

Bombardier BD700-1A11 Global 5000 (source)
Airbus A320 Flight test (source)
Bombardier CRJ700 (source)
US Navy T-45A (source)
Airbus A330 (source)
Boeing B747-8 (picture by Gregor Schläger, source)
Boeing B777, RAT extension switch (source)

Interesting that the Airbus RAT is prevented from deploying at <100KCAS. I'd think a complete engine failure at that speed would only happen at landing, and you'd desperately want any power you could get at that time.

@FreeMan, for one thing below 100 KCAS the RAT won't provide much power anyway and for another 100 KCAS is barely above full flaps stall speed, so you can't be flying at less that 100 kt.

@JanHudec & mins, arguments make sense, but when the RAT deploys, you're already in some extraordinarily unusual conditions, and I'd think you'd want any help you can get. Your reasoning is accepted, just not what I'd expect.

@FreeMan: Just trying to guess: during T/O, 100 kt means you are still rolling on the ground below V1, and T/O abort is likely the best option. During landing, below 100 kt the a/c is stalled, so adding drag will not help. just a guess. Also before RAT is needed, you have the battery powering the critical devices.

@FreeMan, the first thing you want to do when your engines fail is to push down to accelerate to the best glide speed which is something like 205 knots at flaps 1+F and at least 220 knots when clean. The batteries have enough energy to keep ELAC1 and ECAM alive and the hydraulic accumulator has enough pressure to move the elevators to get you there.

Batteries & hydraulic accumulator - makes much more sense now! Thanks!

@FreeMan, indeed. The RAT takes some time to deploy and spin up, so you need batteries to bridge the gap anyway. The reason for RAT is that batteries wouldn't have enough power for the around half an hour you can be gliding down from cruise altitude (Air Transat 236 glided for 29 minutes).

@JanHudec: Also I think many new airplanes can start the APU in flight (if there is some fuel remaining in the tanks, of course). And I read (but there was a controversy) the engines can be left windmilling to produce some energy when shutdown.

@mins, APU has maximum altitude that is in most cases lower than typical cruise altitude and many of the all-engine-out cases were because of lack of fuel anyway. Windmilling engines can indeed produce some hydraulic power (the A320 training manual mentions it), but for jet engines it requires speed that is too high for efficient glide.

There is also no maximum RAT deployment speed on the Embraer 170/190-series commercial jets.

@Porcupine911: Thanks, I've included a reference to E190 AOM.

@mins: And some aircraft have the APU (if working) left on throughout flight anyways.

The rate of spin of RAT in the no-load condition is closely related to TAS, so by comparing this to IAS, you can calculate altitude, or at least density altitude.

I'm not the downvoter, but rotational speed is commonly kept constant by a speed control governor, loaded or not. There is also a mechanism to prevent overspeed and blades damage/release in case of governor failure. If there are exceptions, which is always possible, please provide some reference.


text:
Why do most GA aircraft not have vortex generators?
It is my understanding that vortex generators produce more favorable aerodynamics by delaying flow separation. If this is the case, why do more GA aircraft not have vortex generators?

The vortex generators are used in commercial aircraft (Boeing especially), for a number of reasons, which are usually not applicable to GA aircraft. For example, vortex generators are used for anything from stall alleviation to vibration reduction to noise reduction, some of which are not applicable for GA aircraft. Also, they are usually added during testing, which is much more expansive compared to the GA aircraft.
That said, vortex generators are found in some aircraft, like the Piper Malibu Meridian, as shown below, because of their advantages.

Image from www.nasa.gov
Another point is the a number of companies offer retrofit of VGs for GA aircraft. A number of GA aircraft have been retrofitted with them.

By Ahunt at English Wikipedia - I took this photo and release it to the public domain, Public Domain
However, they are quite costly and require extensive tests for certification, with the result that they are not used much compared to commercial aircraft.

"quite costly" is relative. 1500 USD for a kit and 8-10 hours to install on a 100K  USD airplane to allow for slower landings into a short runway can be quite a worthwhile investment.

Vortex generators are a kludge - a good design does not need them.
They are mostly added at later stages of the design process when some deficiencies need to be cured, but re-designing the aircraft is too expensive. They add a lot of drag, and do so always, for the whole duration of a flight, when they are needed only in some corner of the flight envelope (say, when flying close to stall).
A clever way to install vortex generators can be found on the Short Belfast:

Tail of the Short Belfast from below (own work). Note the vortex generators hidden in the hinge gap - they only move out into the airflow when they are needed: When the elevator is moved to large negative deflections.

Depends on what you mean by "good design".  A typical GA airplane is a generic design, intended to do a lot of things acceptably well.  (And the design probably dates from an era when gas was cheap.)  So if you want to use your particular C-172 in mostly short/soft fields, VGs that lower stall speed optimize the design for that particular use.

Maybe retractable slats might be a bit more efficient, combined with a thinner wing.  The drag penalty would be much less.


text:
How can air travel be so safe if the aircraft skin is only 1-2 mm thick?
Aircraft skin is not very thick at all, yet air travel is the safest form of travel. All aviation professionals know why, but most occupants of an airliner are not professionals. How can it be explained to members of the general public that 1 mm is enough to:

Keep the air pressure in.
Keep the wings from breaking off.
Transport the passengers in greater safety than when they travel in their car.

I've used examples like "you can stand on a full beer can but not on an empty one". What are the secrets of aeroplane construction that makes such a seemingly flimsy construction so safe?
Update
How is a stressed skin construction explained to the general public? Lots of non-experts get quite alarmed when they are thinking of crushing a beer can.

That is a leading question.  What leads you to believe that it is  not possible?  What is it about 2mm thick that leads you to  believe otherwise?  it's possible because 2mm is all that is needed.

Related : https://aviation.stackexchange.com/questions/12200/are-modern-airliners-still-stressed-skin-aircraft

@ Simon I remember telling my sister about 1-2 mm skin, her turning green, and then having to talk for a long time before she appeared to not be unsettled again. How would you explain to someone not familiar with aircraft construction?

Most aircraft skin is not that thick. For example the Lunar Module, the thing that carried astronauts to the moon, had skin that was only 0.38mm thick (0.015").

I genuinely don't understand.  Unless you are able to  say "2mm seems not enough because xyz", without stating what xyz is, how can the answer be anything except "it's only 2mm because that's all that is needed".  Most car body panels are thinner than that.  Why is it safe to get into a car?

Q: If black boxes can survive anything, why not make the whole aircraft from the same material? A: The roads aren't wide enough for aircraft to drive on. In other words, the aircraft has to be light enough to actually fly. Any form of engineering has trade-offs, and weight vs. strength is one of those decisions. But as you said yourself, in commercial aviation the decisions have obviously been very successful from a safety point of view. (Not to mention fuel economy, construction costs etc.)

@Simon car travel is less safe than air travel, isn't it? Yet you very seldomly hear about people with Fear Of Driving.

@Koyovis I do, when I'm the passenger and my wife is driving! :)

Your profile says that you are an aerospace engineer, didn't they cover this in your structural engineering courses?

Yes, and that's exactly the point.  it seems like what you are actually asking is "why do people have a fear of flying".  This is an irrational fear, since if you apply logic and rationale, then the only possible conclusion is that the safest part of any journey involving flying begins when you get on the aircraft and ends when you get off it.  I cannot understand why a person who is not rational would be worried about the thickness of the skin unless they explain why.

Another fun fact: before they used aluminum, an airplane was wood and cloth. :-)

Stand on an empty beer/soda can and show them how much weight it can support. Then, while standing on it, quickly poke in both sides with your fingers and watch how quickly it collapses. Now, build a new empty can with an internal support structure (just like an airplane's) and show that it won't collapse despite a dent in the skin. That's why airplanes are so tough/safe despite the fact that their skin is so thin.

Re "How is a stressed skin construction explained to the general public?" The answer, of course, is that it's not, any more than the general public understands the physics of a cell phone or an LED.  That's what engineers are for :-)

@jamesqf Yes you're right, as long as aunty does not know that there is only 1 mm between her and the troposphere on her flight to Ibiza, she'll be perfectly fine.

@Koyovis: And she'll be perfectly fine if she does know, too.

@Koyovis: But a big part of the reason why car travel is less safe than air travel has to do with all the other idiots on the roads.  If airplanes had the same percentage of mid-air collisions as cars have multi-vehicle accidents, it'd be a much different story.

@jamesqf Indeed. On the other hand: per collision, an aircraft carries much more persons, travelling at speeds and altitudes that cars rarely reach.

@Koyovis But even in those conditions, stats for air travel show it to be significantly safer. Older article: http://traveltips.usatoday.com/air-travel-safer-car-travel-1581.html - In 2008: odds of dying in a motor vehicle accident = 1 in 98 for a lifetime; air and space transport = 1 in 7,178 for a lifetime.

I'll add another comment so that this set of comments is flagged for automatic removal.

And another one.

Simple maths. Cut a cabin in two, lengthwise, and check the forces involved. The halves are held together by 4mm of aluminium, and need to withstand a pressure differential wrt to the outside over the diameter of the cabin.
Let's assume that we have a wide body plane of 6m diameter (larger than a Boeing 777), and a pressure differential of 1atm (of course, usually a plane only maintains up to 0.8atm and does not fly in a vacuum, but let's have some safety margin).
The resulting tensile stress is then about 150MPa. Aluminium only yields at 275MPa.
Other forces are carried by struts, spars and in the case of the fuselage, stringers. The skin transfers shear forces between those perpendicular stringers, but I do not think those forces are significant compared to pressure loading.

"Anywhere between 0 and 1 atmospheres"

•Keep the air pressure in. >> How thick is the skin of a balloon? That keeps in a lot more pressure than an airplane.
•Keep the wings from breaking off. >> That's a little more than just the skin. There are supporting beams to hold the bulk, but wrapping them in the skin strengthens that as well.
•Transport the passengers in greater safety than when they travel in their car. >> Practice, practice, practice. It's only takes a few hours to teach someone how to fly an airplane. But the hours and hours of training and flying after that are what builds a safe pilot who can properly deal with adversity. Plus, there's a lot less congestion in the sky than on the roads.

I don't think a balloon has more pressure than an airplane. Up to 8 or 9 psi is common for a pressurized fuselage.

Your last point is good. Most of the safety is operational. Both airplanes and cars are very mechanically reliable.

@foot Yup. You are correct. It takes >1psi differential to inflate a balloon, which means about 15-16psi to overcome the elasticity of the balloon. But once inflated, the internal pressure is only about 1 atmosphere, with a lot of that attributed to said elasticity. However, that air isn't being let out like it is in an aircraft pressurization system. You aren't really inflating the aircraft; since you are climbing and decreasing the outside pressure, you're trying to keep the inside pressure at a human-compatible range. Regardless, your inside pressure ...

...will be higher than outside pressure, so your airplane would have a bit more pressure inside. I guess my point would be more comparable to the inflation of the balloon. The skin of the balloon can withstand a good bit more internal pressure during inflation without bursting. But an aluminum skin has a lot less elasticity. I kinda oversimplified a somewhat complex pressurization issue. :-/

Maybe you should add to your last point systematic incident and accident analysis and continuous training (in many country you pass your driving license once and it is valid for your entire life whatever the evolution of your car or your health)

Your balloon analogy is false, as is your explantation in the comments. Elasticity is of negible influence in stresses in a thin walled pressure vessels (although not in a balloon, due to large deformations and high Poisson ratio in rubber). Max pressure differential in a balloon is about 0.7 psi. Finally, the aircraft skin does not care about whether air is "let out", only about the pressure differential between inside and outside.

"I kinda oversimplified a somewhat complex pressurization issue." Plus, I wasn't very clear in my explanation, I guess. 14.7 psi (sea level pressure) + about 1 psi to inflate it = 15-16 psi inside the balloon. I don't think the balloon analogy was false; just not as applicable as I originally thought. There is a bit of physics involved in balloon pressure. However, aircraft skin absolutely does care whether or not pressurization air is "let out". Since new air is constantly coming in, the old air has to be let out. That's how the pressure differential is controlled. If you continue to put ....

... air into a balloon without letting some out, it will eventually burst. So I guess a more appropriate experiment would be to measure the pressure inside an inflated balloon as it approaches the bursting point. And that would depend on the balloon's skin. Again, that's a lot more physics than the initial simple explanation I assumed.

@Shawn Unless you're planning on taking a balloon to high altitude (where it will inevitably burst), I still don't see how the analogy holds - the balloon does not care about absolute pressure, only about the difference between inside and outside. And I meant to say that the skin does not care if a certain pressure is achieved through a continuous inflow or by shutting the outflow. I think you're not oversimplifying, but the opposite; balloons are actually a very tricky, highly nonlinear problem, behaving very counterintuitive (blowing in more air decreases the pressure!)

@Sanchises I think we may be getting way off into the weeds here, and maybe I misunderstand the physics of inflating a balloon, but when any pressure vessel reaches the limit of its elasticity, it will burst if the pressure differential increases and the strength of that vessel's walls is exceeded. Is that not why a high-altitude balloon will burst if it keeps going? The skin cares that the pressure differential is greater than it can hold in.
...

Regardless, atmo pressure at sea level is about 14.7 psi. If you're flying at 36k ft, atmo pressure is 3.3 psi. If you pressurize your aircraft cabin to 7000 ft, that's 11.3 psi inside the aircraft. Differential is 11.3-3.3 = 8 psi. So 8 psi is being exerted on the pressure vessel.
I'm not sure what additional pressure it would take to make a balloon burst, but I doubt it would get to 8psi before burst pressure is reached. I was wrong above. A balloon doesn't really make a good example. And I can't think of another way to explain a thin-walled pressure vessel.

http://aerosavvy.com/wp-content/uploads/2015/05/altitude-graph.jpg

@Shawn Exactly. My whole point was about "Keep the air pressure in. >> How thick is the skin of a balloon? That keeps in a lot more pressure "; so I think we're on the same page now. I can't really think of an intuitive example for a pressure vessel either - perhaps a piece of paper loaded only in pure tension is a passable example (try and rip that apart without tearing). Other small pressure vessels (boilers, etc) are generally very over-engineered to account for out-of-plane loading (dents, etc).


text:
What is ILS 114 / 162 / 294?
I am searching for airports that have ILS (not sure if there is a "law" that any airport servicing civil flights must have one).
So I googled around and found this nice site http://www.azworldairports.com.
So I've been looking at airport in Vienna and in airfield section there is following:

Runway 1: Heading 11/29, 3 500m (11 482ft), 75, ICAO Cat. I, 3B, Aircraft size max: E, Rwy 11, ILS 114, Lighting: 11 CAT 1/PAPI, No Restrictions, Rwy 29, ILS 294, Lighting: 29 CAT 1/PAPI, No Restrictions
Runway 2: Heading 16/34, 3 600m (11 811ft), 75, ICAO Cat. I, 3B, Aircraft size max: E, ILS 162, Lighting: CAT III B/PAPI, No Restrictions

What exactly is ILS 114 / 162 / 294?

"Not sure if there is a law that any airport servicing civil flights must have one." Oh no, that is certainly not the case.

@J.Hougaard Indeed. The vast majority don't have it.


text:
What is this "stick" on top of Cessna 152's wing, and how does it work?

What is this device? What is it for? Seems that some variants have only one, while some others have two.
Added: As some have two, so what are they? The answers and comments have not explained which antennae they are, especially for the ones with two antennae. As I asked, how does it work?  I need an explanation; if the two antennas are for the same purpose, then why do they need two at the same place?

They are radio antennas. Most airplanes have two radios but small trainers like these often only have one.

What antenna are they?

Communication radio antenna.  118.00 to 136.975 MHz in 8.33 KHz spacing.  Example: https://www.aircraftspruce.com/catalog/avpages/domeant.php

Two antenna for two radios. Pilot can talk/listen on one, copilot can talk/listen on the other. Or pilot can talk/listen on one, and monitor incoming announcements on the other, such as automated weather & traffic advisories for the intended airport of landing.

Or, is it Tx and Rx antenna for the ones with two antennas?

@CrossRoads, why don't you put your comment in comment column so I can accept the answer? But please also include the answer of my question above so it will be more comprehensive.

Huh? I would have Answered, but that is not a choice.

Short answer: They're antenna(s) for the onboard VHF radio(s).
Airplanes aren't actually required to have radios. But, if you want to fly into certain classes of airspace, you must have at least one VHF radio on board, to communicate with air traffic control. A lot of planes (in fact, I would say the majority of them) actually have two radios installed. Thus, you see planes with zero, one, or two antennas.
Having two radios has several advantages, beyond just the obvious "backup in case one fails" advantage. Most often, one will be used to listen to the ATIS/AWOS/ASOS or other automated information broadcast while the other is used to keep in touch with ATC. You could also use the second one to contact Flight Services, or (if you're charter/commercial) keep in touch with your company (although that last one is getting more and more likely to be done with a cell phone or the internet).

Thank you for your explanation.


text:
What are the advantages and disadvantages of tail wheels that cause modern airplanes to not use them?

Photos' source: Cessna 140 and Cessna 150.
According to Wikipedia here, the Cessna 150 is successor of Cessna 140. Both have two-seat capacity and a single engine.
Why was the Cessna 150 changed to tricycle landing gear from the "old school" tail wheel? If we consider that the two airplane models have the same weight and capacity, then what are the advantages/disadvantages of tail wheels vs. tricycle landing gear? Why are so few new tailwheel aircraft produced?
Note:
I mentioned the Cessnas here solely because I knew their story better, so I can more easily compare them. This question should not be considered specific to only Cessna's products.

Clarification please - do you intend to aim this question at light aircraft like the Cessnas pictured?

@Criggie, not for light aircraft only,but for all size. But both the light and the heavy aircraft, all are tricycle now, right?

WRT commercial planes, I'd guess that it's as much a matter of passenger convenience as anything.  Consider how much worse the boarding & deplaning process could be with a tilted floor :-)

Related: Why tail wheel rather than tricycle? and Why do most aerobatic aircraft have conventional landing gear?

A tailwheel is a good choice for operation on unprepared surfaces with aircraft that have low wing loading and need to be as light as possible. Two main wheels and a small tail wheel weigh less and cause less drag than a tricycle gear, especially if they cannot be retracted. On the Bo-209 Monsun only the nose gear was made retractable because, being positioned right behind the prop, it caused 40% of gear drag all by itself.
On the other hand, high wing loading aircraft with their high landing speed would need very long landing runs due to their inability to brake hard. A rejected take-off close to the decision speed would be impossible without crashing through the airfield perimeter. Braking too hard with a tailwheel configuration will cause a headstand.
To summarise the reasons given in this answer:

A tricycle gear offers better visibility on the ground.
A tricycle gear allows full brake application.
A tricycle gear makes loading and unloading easier because the fuselage is horizontal.
A tricycle gear has less drag during the initial stage of a take-off run.

Cessna simply shifted priorities between the 140 and the 150.

40% isn't excessive when if balanced, it would cause 33%...

@Harper, 40% is excessive when you realize the nose wheel is generally much smaller than the main gear.  In general, a tricycle-gear plane is balanced so that the center of gravity is just barely in front of the main gear.  The nose gear supports just enough weight to keep the airplane from tipping over backwards.

Thank you Peter for the nice explanation. Very complete.

I'd think another advantage is that you only need one wheel to turn for steering, while the tail-wheel model has two steerable wheels, which means more mechanical parts, thus more weight in order to steer the plane while taxiing.  You could instead design it so that the rear wheel steers, but that's far less intuitive if you're not used to it, and probably mechanically more complex - thus heavier - to run controls all the way down the tail for that.

@Darrel Hoffman: AFAIK the main wheels on taildraggers aren't steerable.  Can't say there aren't any, but the ones I've seen aren't.  Even in a tricycle gear light plane, you steer as much or more by differential braking of the mains as by turning the nose wheel.

@jamesqf I'll trust your experience on this one then - I'm not a pilot, just a programmer who has worked on aircraft maintenance simulators for various military planes, which tend to be considerably larger than Cessnas of course.  At least some of them definitely have steerable nose-gear.

@DarrelHoffman: You need to run control cables to the rudder anyway, so adding a steerable tail wheel is not much of a complication. Main wheels are always not steered, and even most tail wheels (and all tail skids, obviously) are not steered, but free castering. Rudder is used to swing the tail around. On very early designs like the Blériot XI, even the rear wheel could not caster, so the tail had to be lifted up and carried around to have the aircraft point into the right direction.

@Darrel Hoffman: WRT nose wheel steering (at least on small plane), it's not so much a question of either steering with the nose wheel or with differential braking, but of using both.  If you're just rolling down the taxiway, you might be steering with just the nose wheel.  If you need to make a tight turn, you brake on one side.  And of course if you're moving at any speed, you still have some effective rudder.  But they're all linked, and really you tend to forget about exactly what is going on - or at least I do.

What did Cessna call it? Something like "autoland" when they switched to tricycle.

@tedder42 Cessna’s marketing dept called it Land-O-Matic.

A tailwheel aircraft is particularly susceptible to a dynamic instability during landing which causes the plane to violently spin around, point backwards, and skid off the runway. This is called a ground loop and is one of the leading causes of landing accidents in tailwheel aircraft. Avoiding ground loops requires good reflexes, good training, and lots of practice.
A tricycle gear aircraft is immune to ground looping, making it easier to handle on the runway during landings.
Because the tailwheel aircraft has no nose gear, it will weigh less and experience less drag during flight than the same airframe with a nose wheel, so it can fly a little faster and a little farther on the same fuel.
However, the cost to insure the tailwheel aircraft against landing accidents is greater than the cost to insure the tricycle-gear plane, which wipes out any savings on fuel burn.

Thank you for your nice explanation. It helped me understand better.

Not completely immune; I managed to ground loop a tricycle plane during runup when one of my brakes failed. Glad it happened then and not during landing!

Already some excellent answers here, but to add to Peter's response, in spite of the challenges, some folks prefer tail wheel planes because they are better for back country type flying - landing on unpaved surfaces, etc. If you take a look at any of the short take off and landing (STOL) contests around the country, you see almost exclusively tail wheel planes competing.
AOPA article on STOL contests

Thank you my friend for your explanation. Now I know better where it can be used.

Why was the Cessna 150 changed to tricycle landing gear from the "old school" tail wheel?
According to my old-time teacher, the accident rate in flying school changed dramatically when changing to tricycle gear. Ground loops used to be common and potentially expensive with tail-wheels. That seems to be enough of reason for a flight school to stop buying tail wheel planes. ( The 150 I believe, was mainly targeted at schools ).
For specialized uses with trained and experienced pilots, tail-wheels are still in use.

I am sorry. Seems that you have misunderstood my question. "Old school" means is, "old time". So, this question is not related to flight school. But however, thank you anyway for your explanation.

The point is that the main buyrrs of the 140 and 150 used to be flying schools.

Conventional gear is the term that includes both tail-wheel and tail-dragger, because there may only be a simple skid in place of the tailwheel. It is simple to construct, lighter and lower cost for a given strength than tricycle gear.
Tricycle or nose-wheel gear. The main gear is very similar to conventional main gear it is just moved slightly behind the center of mass so the plane tips nose down instead of tail down while on the ground. But a wheel is added to the nose of the airplane to prevent it tipping forward.
The nose wheel mechanism is much more complex than a tail-wheel caster, with much more mass and some inherent weaknesses on rough surfaces.
If nose gear is broken by a hole or bump on the ground the plane tips forward into the propeller, while a conventional gear plane can land with little damage even if the tail-wheel had fallen off completely. The nature of the nose down moment caused by obstacles during the landing roll takes load off of a tail skid/wheel and adds load to a nose-wheel. Also when you look at the wheel/skid path taken during a pitch change(caused by hitting an obstacle) the nose wheel tends to catch and dig in causing a huge rise in forces while a rear skid/wheel has a more stress relieving motion. As an illustration take a long pole or stick and drag one end on the ground behind you then turn around and try to push it, you see how the tip catches on everything when pushed over a rough surface and slides with little effort when pulled.
There are 3 primary disadvantages of conventional gear. First is that the center of mass is behind the main gear which is dynamically unstable while turning on the ground. A small turn to the left wants to grow into a large turn to the left so the pilot cannot be passive and must actively correct any unwanted deviations. Any vehicle with rear wheel steering has this stability issue but sometimes the trade offs are worth it, note most forklifts and many commercial mowers have rear wheel steering.(mainly for strength and maneuvering)
The second disadvantage is the nose high position while on the ground which creates very poor visibility for the pilots during taxi and takeoff roll. For jet aircraft in particular it directs the jet blast at the ground which creates a debris hazard and can even damage and lift pieces of pavement.
The third disadvantage for conventional gear is that they cannot utilize all of the theoretically available braking friction on dry pavement, because braking force is limited by forward pitching moment, so landings and aborted takeoffs rely more on aerodynamic drag like flaps and high angle of attack and elevator down force to resist brake pitching force. On dirt and grass the maximum available braking friction is much lower than on dry pavement so the tricycle gear does not gain substantial advantage in maximum braking force. On snow or ice, tire friction limits the maximum force more than the pitching moment, and so the tricycle has no braking advantage and conventional gear is lighter and easier to fit with skis.
However, in practice with small aircraft most very short landings are unpaved surfaces and most paved runways are long enough that small aircraft don't actually need wheel brakes and pilots will avoid using the wheel brakes to reduce the maintenance costs. The braking distance issue is thus not a primary consideration for many owners. Most pilots also tend to maintain a high angle of attack during the landing roll regardless of the gear arrangement, which adds lift taking weight off of the wheels and so prevents maximum tire friction. On these aircraft, the lack of braking force is somewhat compensated by light weight enabling a slower landing speed and generally less kinetic energy, further reducing the braking advantages of tricycle over conventional.
With large aircraft, especially jets, braking distance is a common issue, and they are often fitted with devices like spoilers to stop the wing from making lift and to get maximum friction from the tires right at touchdown even with a high angle of attack.
These aircraft never operate on dirt, grass, or deep snow, the runways they use are inspected for holes and objects multiple times per day. The weight and cost advantages of conventional gear are less substantial when adding retractable landing gear mechanisms, and the high speed drag advantage of retraction is far greater than the weight advantage of any type of fixed gear. So the advantage in this category is very much in favor of tricycle gear.

A fourth disadvantage of conventional gear is that the nose-high stance on the ground can be problematic in the case of a strong or gusty headwind component.


text:
Do airplanes need brakes in the air?

A few days ago I flew on a Boeing 737-900ER. Luckily I was sitting just beside the left wing, seat 26B. I observed that the spoilers were activated (opened) just after the pilot spoke on the Passenger Address System announcing that the airplane was preparing for landing (if I was not mistaken) and began to descend. The spoilers then opened for around 3-5 minutes (picture 1) and then closed again (picture 2) until it touched down. In flight they were not as fully activated/opened as when the airplane landed.
My question:

What is their purpose? Are they brakes?
If they are brakes, does an airplane need brakes in the air?
If they are brakes, is not more efficient to reduce the engine's power rather than to keep it constant power but brake to slow down?

WRT your third point, sailplanes are commonly equipped with speed brakes and/or spoilers, and they don't even have engines.  It's a question of whether  aerodynamic drag will slow the plane quickly enough with no power applied.  With the sailplanes I've flown, without using speed brakes you'd float all the way down the runway in ground effect.

If you listen, you will notice that at that time, engine is already at idle.

@user3528438: But then the question arises: "is it not more efficient to reduce the engine's power earlier?"

@phresnel That is explained already in the currently accepted answer. E.g. Air traffic control gives you a shortcut., which you didn't know at the time you would have had to idle the engines

@PerlDuck: I know. I just wanted to show that the comment I replied to will not satisfiably answer AirCraft Lover's question or just delays the real answer. But then, it's just a comment anyways.

the most fuel efficient way to drive your car is to never apply the brakes and cut the engine far enough that your car will slow to a stop as it enters your parking space. What's your point?

@BooleanCheese I did that as a poor college student!   Not literally, but as much as possible anyway, and especially in stop and go traffic.

I am not an airline pilot, but I am a glider pilot. Despite the name, airbrakes or speedbrakes don't change your speed. Attitude does. It really depends on the glider but airbrakes are actually just ruining your performance, increasing drag and thus burning energy quicker. They are not used to control speed. If however the spoilers have an aerodynamic effect so that the pitch changes, the speed might change, but this happens rarely as the position of the airbrakes is generally very near the center of lift. Actually, if you were too fast, and you opened airbrakes, you'd probably break the wing.

"I’m gonna hit the brakes, he’ll fly right by" - Maverick

I seem to always get the window seat in this vicinity and am a frequent spectator to this sub. Thanks for asking this one!

@Stefano Borini: Yes, you do use speed brakes/spoilers to slow down (or at least I would), in the same way that you would use a car's brakes to slow down when descending a mountain road.

@jamesqf that does not happen in my glider. if I open the airbrakes, airspeed stays the same, and the sinkrate increases. It could be a result of design though

What you saw is called a speed brake, which is one of the functions of the spoilers. From the Boeing 737 NG FCOMv2 (9.20.5 Flight Controls - System Description):

Flight Spoilers
Four flight spoilers are located on the upper surface of each wing. Each hydraulic
  system, A and B, is dedicated to a different set of spoiler pairs to provide isolation
  and maintain symmetric operation in the event of hydraulic system failure.
  Hydraulic pressure shutoff valves are controlled by the two flight SPOILER
  switches.
Flight spoiler panels are used as speed brakes to increase drag and reduce lift, both
  in flight and on the ground. The flight spoilers also supplement roll control in
  response to control wheel commands. A spoiler mixer, connected to the aileron
  cable-drive, controls the hydraulic power control units on each spoiler panel to
  provide spoiler movement proportional to aileron movement.
The flight spoilers rise on the wing with up aileron and remain faired on the wing
  with down aileron. When the control wheel is displaced more than approximately
  10°, spoiler deflection is initiated.
Speed Brakes
The speed brakes consist of flight spoilers and ground spoilers. Hydraulic system
  A powers all four ground spoilers, two on the upper surface of each wing. The
  SPEED BRAKE lever controls the spoilers. When the SPEED BRAKE lever is
  actuated all the spoilers extend when the airplane is on the ground and only the
  flight spoilers extend when the airplane is in the air.
The SPEEDBRAKES EXTENDED light provides an indication of spoiler
  operation in-flight and on the ground. In-flight, the light illuminates to warn the
  crew that the speed brakes are extended while in the landing configuration or
  below 800 feet AGL. On the ground, the light illuminates when hydraulic pressure
  is sensed in the ground spoiler shutoff valve with the speed brake lever in the
  DOWN position.

You noticed that the spoilers did not extend as much in the air as on the ground. This is intentional to reduce buffeting:

In-Flight Operation
Operating the SPEED BRAKE lever in flight causes all flight spoiler panels to rise
  symmetrically to act as speed brakes. Caution should be exercised when
  deploying flight spoilers during a turn, as they greatly increase roll rate. When the
  speed brakes are in an intermediate position roll rates increase significantly.
  Moving the SPEED BRAKE lever beyond the FLIGHT DETENT causes
  buffeting and is prohibited in flight.

The speed brake lever must not be moved above the FLIGHT DETENT while in the air. You can see it in the following image from the FCOM:

To answer your other questions:

If it is brake, do an airplane need brake in the air?

Yes, sometimes. Ideally a flight profile is calculated that does not require the use of brakes in the air. That means the Flight Management Computer (FMC) calculates a so called Top of Descent point at which you can start an idle descent. So the engines are retarded to idle thrust and the aircraft maintains the target speed by descending. This will get you on final approach at the desired altitude. Two things can happen now:

Air traffic control does not allow you to descend when you want to. This usually happens when there is conflicting traffic and you have to stay at a higher altitude than the idle descent profile for a short while. Afterwards you need to descend faster, which may require the use of speed brakes. You can also try to just increase the target descent speed (since higher speed means more drag and gets you down faster), but if that is not enough speed brakes are needed.
Air traffic control gives you a shortcut. This happens quite a lot. Some Standard Terminal Arrival Routes (STARs) involve some complex maneuvers and turns, but if traffic allows you can get a shortcut. Now your distance to the airport has just been reduced, requiring a steeper descent. Again, use of speed brakes helps here, especially when you are close to the airport already because below 10,000 ft you have to maintain 250 kt or lower, so speeding up is not an option.

If it is brake, is not more efficient to reduce the engine's power rather than to keep it constant power but brake to make it slower?

The engine will already be reduced to idle on a normal descent profile. Therefore using less power is not an option any more. Use of reverse in flight is usually prohibited for most aircraft, e.g. 737 FCOM:

Intentional selection of reverse thrust in flight is prohibited.

What does it mean, "prohibited"? Does that mean it's possible?

@Harper It should not be possible. From the FCOM: "The thrust reverser can be deployed when either radio altimeter senses less than
10 feet altitude, or when the air/ground safety sensor is in the ground mode."

I know nothing of aviation, but I think in principle he is right that using idle earlier could save more power. In the same sense, a train could avoid breaking if it went on idle with so much advance as to reach speed 0 exactly at the station. The problem is, most of the time you want to get from A to B in the shortest possible time. In this case, the optimal controls have a phase of max acceleration, cruising, coasting, maximum breaking. This can be proven using Pontryagin's maximum principle.

@AlbertoSantini Absolutely correct: whenever you use speed brakes, you basically waste fuel because you could have started an idle descent earlier (that's why the speed brake lever is sometimes called the lever of shame). The problem is that you didn't know that earlier in the cases I mentioned.

@Bianfable can we say that even trying to deploy thrust reverser in-flight is prohibited? Wouldn't it cause abrubt nose-down movement and irrecoverable descent?

@Crowley Yes, that is prohibited. As far as I know it is prohibited for every airliner. Concorde was somewhat special: they needed thrust reversers for a fast descent in case of loss of cabin pressure, so two out of four engine reversers could be selected in flight. I don't think it creates an irrecoverable descent, but if the reverser cannot be disengaged any more, the aircraft is basically un-landable.

@Bianfable Well, Concorde is an extraordinary plane. In this case it can profit from the position of the engines. Now typical airliners with engines quite far down from the centre of mass will get hard kick in the nose direction though.

Thank you for the comprehensive explanation.

@Bianfable DC-8 and Hawker Siddeley Trident were two jet airliners capable and certified for deploying reversers in-flight. However even with those it was rarely done in practice, due to the heavy buffeting it caused.

Flying is not about saving fuel (there's enough fuel on the airplane, and you could save all of it if not taking off in the first place), but about arriving quickly (otherwise you could just walk). "Ideally" should be reworded to include "in terms of fuel economy".

@Haukinger It is about both (actually, most aviation is about money from tickets) and the company economics determines the cost index preferred, not an arbitrary choice of the pilot. Time in the air costs money, fuel costs money, it is a difficult process to find the optimum, but it is very important. Private flights may have different priorities but with airliners they are the minority. Normal airlines do not fly or cruise as fast as possible.

@Harper "Prohibited" as in "to certify the aircraft you have to demonstrate that it can't happen". Of course things that are have been "proved" to be impossible do happen sometimes - usually not in a good way. https://en.wikipedia.org/wiki/Lauda_Air_Flight_004.

Can autopilot deploy speed brakes if there's a need for them?

@frizzby No, I don't know a single aircraft where the autopilot would have control over the speed brakes. They are all manually controlled. There is actually a rule in most airlines that the pilot flying must keep their hand on the lever to make sure they don't forget that they are deployed.

@AlbertoSantini, unlike train, where braking later means getting there sooner, an aircraft can keep its forward speed until relatively late in the descent, so there is very little time to gain, so speed brakes are only used to avoid other traffic, or sometimes terrain, or if the pilots misjudged the descent, but that is rare now that FDMs can calulate it. Or sometimes if the wind changes, the aircraft gets rerouted to other runway so the track is shorter and they are already a bit past the top-of-descent for the new track.

Though your photos refer to a specific commercial airliner, your question is general, so I will describe a more specialized use case for speed brakes. A glider (also known as a sailplane) has no engine, and it is designed to have a very high glide ratio so that it can travel further between sources of lift. If a glider were to come in for a landing without using speed brakes, it would descend at a very shallow angle, starting far from the airport. A long, shallow approach is dangerous because the aircraft will be too close to the ground far away from the airport; also, lacking an engine, the glider has no ability to increase its speed and altitude simultaneously to avoid an obstacle. Further, when starting at a long distance from the airport and a low altitude, it would be virtually impossible for a pilot to judge the correct glide path to land at the desired point. Gliders deploy speed brakes or spoilers to enable a steeper glide path during the final approach. If an emergency occurs, such as a last-minute obstruction on the runway, the glider pilot may retract the brakes to gain speed or altitude, to gain additional time on approach.
Note that a pilot can't simply point the nose of the aircraft up or down to change the landing point, because that would change the airspeed of the aircraft. The aircraft's airspeed needs to be within a specific range in order to touch down safely.

And with many GA planes, you get much the same effect with a slip.

As you can see on the photo, the flaps are slightly extended, allowing the aircraft to fly at a lower airspeed. This is common for aircraft to do during an approach, to keep a constant distant between incoming flights. An aircraft flying into an airport, can perform a continuous descent or a step down decent. Illustrated in the figure below:

From the figure it is seen that the for the step down approach, the aircraft needs to move relatively quickly from one flight level to the other. The pilot can do two things to achieve this, put the nose down, increasing airspeed and giving passengers a rollercoaster ride! Or, the other option would be to temporary reduce the lift of the wings without affecting the airspeed that much in order to keep a given separation between consecutive flights.
The spoilers on the wing you see deployed are also called lift-dumpers and are used to, while being extended, reduce the lift of the wing and thus have the aircraft "falling" out of the sky in a controlled fashion. They also reduce the airspeed of the aircraft due to increased drag, thus often as the spoilers are retracted again you notice the pilots increasing the throttle. In short, the lift dumper are used to move to a lower flight level in a controlled fashion without the need to push the nose down.

In your diagram, the slopes of the descents in the step-down profile are less than or equal to that of the continuous descent.

Yes, deploying the speedbrakes makes for a steeper descent - can be seen quite often.

Yes, those are speed brakes, used to slow the aircraft down. In this case, they also double as spoilers to kill lift once the aircraft has touched down.
A propeller aircraft can be slowed down while in flight by throttling down and in some cases flattening the pitch on the prop blades... this created a lot of air resistance and slows the aircraft.
When turbine engines, both turbojet and turbofan, were developed for aircraft use, one drawback was that they offered no in flight braking ability like prop aircraft have. No big propeller to create air resistance when the prop is slowed down and/or the pitch is flattened.
That is why the aircraft you were on didn't reduce speed by cutting power... it would have almost no effect on the aircraft's immediate speed. Also, when approaching a field for landing, the aircraft is descending and tends to pick up speed, so the speed brakes are used to keep the aircraft's speed within what is desirable for a landing approach.
Imagine trying to turn off of a downhill highway into a parking lot with no brakes on your car, and you can see how this lack of immediate braking ability could be a problem, especially when approaching an airfield.
So, speed brakes, flaps that would open up to create air resistance, were developed for jet aircraft, so that they could slow down quickly when needed.

The spoilers decrease the wing's lift coefficient.  Yet in the context of flight, as opposed to the context of after touchdown, it's not really correct to imagine that they are forcing the wing to generate less lift.  In a steady-state descent, lift is only slightly less than weight, unless we are talking about a really extreme dive angle.  So what the "lift dumpers" are really doing is allowing the plane to fly at a higher angle-of-attack and thus make more drag for any given airspeed than would normally be the case.  Also, the devices generate drag for other obvious reasons, independent of the increased angle-of-attack of the wing.  The two effects in combination are what allow the plane to descend at a steeper glide angle than would otherwise be the case at the same airspeed and power setting.  Or conversely, we could say the two effects in combination are what allow the airspeed to be lower than it otherwise would be at the same glide angle and power setting.
It's a bit counterintuitive that deploying flaps and deploying a "spoiler" or "lift dumper" or "airbrake" BOTH can have the effect of making the glide path steeper than it otherwise would be at the same airspeed and power setting, i.e. of making the airspeed lower than it otherwise would be at the same glide angle and power setting.  Food for lots of thought on your next long airline flight.
To really start to come to an understanding of this apparent paradox, you'll need to look at how each device modifies the curve of L/D versus airspeed, which is also essentially a curve of sink rate versus airspeed.  But that's probably more than you wanted to know!

authors: Vladimir F Героям слава